Downhole measurement-while-drilling (“MWD”) tools that transmit data uphole using electromagnetic (“EMAG”) telemetry include an electrical insulation layer (e.g., ceramic, hard plastic, rubber) positioned between an upper portion of the tool and a lower portion of the tool. This is typically integrated inside a permanent connection in the collar. To transmit the data stream from within a wellbore to a surface location, a coding method is used: typically, a predetermined carrier frequency is selected and a PSK or QPSK coding is superposed to define the bit pattern. This coded signal is applied as a voltage differential between the upper and lower portions of the tool. Due to the voltage differential, current is generated that travels through the subterranean formation. More particularly, the current travels from the lower portion of the tool, out into the subterranean formation, and bends back toward the upper portion of the tool, in an almost semi-elliptical shape. The current collected by the upper portion returns towards the lower portion by flowing downward through the conductive material of the upper portion.
To receive the signal at surface, two metallic stakes are driven into the subterranean formation at the surface location. When some of the current reaches the stakes, a voltage differential is generated between the stakes, as the surface formation has some electrical resistivity. The voltage differential is applied onto the acquisition system of the surface computer so that a computer system can decode the voltage differential to recover the data stream that was transmitted from the downhole tool in the wellbore. Sometimes, however, the subterranean formation may include one or more layers having a very high resistivity or a very low resistivity that may strongly limit the current from passing therethrough and reaching the stakes. As a result, the signal (e.g., the voltage differential) may be too weak at the surface stakes and the data may not be recovered at the surface location.